Electric systems are gradually replacing hydraulic systems on many commercial, and military, aircraft. Current “brake by wire” aircraft systems may have a generally centralised architecture in which pilot inputs are interpreted and command and monitoring signals are communicated via a databus and as analogue/discrete signals to a brake control unit (BCU). An exemplary centralised architecture is described in US 2008/0030069 A1.
The BCU interprets the commands from the aircraft cockpit controls and avionics and calculates braking force commands for each actuated landing gear wheel of the aircraft. This may include fast loop anti-skid control.
Each braking wheel will have at least one electro-mechanical actuator (EMA) for providing a clamping force to the brake for that wheel, which converts the clamping force to a braking torque. Electro-mechanical actuator controllers (EMACs) may be disposed within the landing gear bay and electrically connected to a plurality of brake EMAs coupled to wheel and brake groups. Typically, each wheel and brake group includes a plurality of brake EMAs coupled via a brake assembly to a wheel. The EMACs interpret the brake force commands from the BCU and receive electrical power to provide power to drive the EMAs.
Typically at least two BCUs are provided. The plurality of BCUs may be arranged for redundancy and/or fault tolerance. In a redundant configuration, the BCUs may be assigned to particular sides, e.g. aircraft avionics network side or electrical power network side. The EMACs may therefore receive brake force commands from any BCU. To maximise commonality of parts the EMACs may all be identical so as to minimise the cost and complexity of design, manufacture, installation, repair, replacement, etc. of parts. There is a therefore a potential for simultaneous failure of several EMACs leading to partial or full loss of braking control, which is undesirable. The EMAC may be considered a “complex” part, i.e. it is not fully testable, as defined in ARP4754.